Light emitting diode (LED) based light-emitting devices are today increasingly used for a wide variety of lighting applications, including for instance office lighting luminaires, downlighters and retrofit lamps. White light may be obtained from an LED by using a blue LED and a wavelength converting material, sometimes referred to as a phosphor, which absorbs part of the blue light emitted by the LED and reemits light of longer wavelength(s). For reasons of efficacy it is preferable to have the wavelength converting material arranged at a distance from the LED. Usually, the wavelength converting material is applied on a substrate, which is for example arranged at the light exit window of the device. However, the adhesion of the wavelength converting material to the substrate often requires the use of a transparent coating film which may decrease the optical efficiency of the lighting device.
Since the light emitted by the wavelength converting material is emitted in all directions, a back reflector is generally used for reflect light emitted back into the optical chamber so that it is redirected towards the exit window. However, to provide a homogeneous white light output, the non-converted light, i.e., the blue light, must be effectively scattered as well. Usually, scattering of non-converted light is achieved by placing a diffuser at the exit window and/or using a diffusing back reflector. The use of an additional optical element, such as a diffuser, with reflections on all surfaces will however lead to a lower light output of the lighting device.
WO 2007/130536 discloses a lighting device which comprises solid state light emitters such as LEDs, a thermal conduction element and a reflective element. The lighting device may optionally include a lumiphor such as a phosphor. However, WO 2007/130536 does not provide a solution to the above-mentioned problem of adhesion of the phosphor.
Thus, there is a need in the art for improved LED-based lighting devices.